When Allies Are Too Zealous

Evolution has forged a tight alliance between East African acacia trees and several species of ant. The ants get a home--inside modified thorns called pseudogalls--and a source of food, produced for them by glands on the tree. In return, the trees are protected by the aggressive, biting ants--which can number in the tens of thousands--from other insects, and probably any hapless herbivore that dares to munch its leaves.

The trees, however, aren’t always well served by their dutiful guards. Acacias, like any flowering plant, rely on bees and other insects for pollination. But how do these bugs get through the tough defenses of the ant patrol? According to zoologists Pat Willmer of the University of St. Andrews in Scotland and Graham Stone of Oxford, they get a helping hand from the acacia tree itself.

Willmer and Stone studied the activity of pollinating insects and the acacia ant guards during flowering season (November to January) on trees in Tanzania’s Mkomazi Game Reserve. Acacia flowers are essentially big, fluffy yellow balls of pollen. When the flowers are closed buds, the researchers found, they are patrolled by ants. But as soon as the flowers begin to open, the ants back off. Because the ants avoid the flowers at only this particular stage, Willmer and Stone suspect that the flowers make some antiant chemical--probably in the pollen, which is right at the surface of the flower.

The ants begin to get alarmed by the flowers, Willmer says, and they don’t go to them at all. With the coast clear, pollinators-- mostly bees--swarm in and quickly remove the pollen. A single flower that opens midmorning will be tapped out of pollen by midday. As the flower starts getting older, she says, the ants return to it and start protecting the flower again from things that might eat it or the developing seed.

As to the exact nature of the flowers’ defensive chemical, we’re still working on it, but it seems to mimic the effects of ant-alarm pheromones, Willmer says. If an ant is attacked, it will secrete a chemical from its body--a pheromone--that notifies other ants in the nest of the threat. If the other ants think there is danger, they will run away.